The invention concerns a subscriber card, a subscriber unit, and an exchange for concentrating Internet frames, i.e. frames conforming to the point-to-point transmission protocol and to the Internet protocol.
Many Internet users employ the public telephone network to access their service provider. Calls conveying data between an Internet service provider and a subscriber terminal via a standard telephone network are longer than standard telephone calls. On the other hand, the data is transmitted in bursts separated by long periods of inactivity.
In an analog telephone network, a subscriber terminal sends and receives Internet frames via a modem located near the terminal and another modem located near a server constituting an access point to a data transmission network.
One particular type of modem, called an ATM/ADSL (Asynchronous Transfer Mode/Asymmetrical Digital Subscriber Loop) modem, transmits Internet frames having a high bit rate in the direction from the subscriber terminal to the network on an analog line in a frequency band above the telephone band.
In an Integrated Services Digital Network (ISDN), basic subscriber access is provided by:                two B channels each having a bit rate of 64 kbit/s, and        one D channel having a bit rate of 16 kbit/s and which is used to transmit data.        
Each ISDN subscriber terminal is connected to a local exchange via a subscriber unit which can be at some distance from the exchange. Subscriber cards in the subscriber unit constitute the interface with the subscriber lines. When a subscriber uses the D channel to transmit data conventionally, the data is concentrated at the subscriber card by means of a circuit called a HDLC controller which implements the standard HDLC protocol, after which the concentrated data frames are switched in order to route them to a gateway providing access to a data frame switch.
When a subscriber uses a D channel to transmit Internet frames, those frames can be separated from frames dedicated to signaling and then routed without further processing, in the local exchange, to a node providing access to a data transmission network.
When a subscriber uses a B channel, or both B channels, to transmit Internet frames, those frames can be routed without further processing, in the local exchange, to a node providing access to a data transmission network. The access node is a remote node. It is located beyond the local exchange, or even beyond one or more transit nodes.
In an ISDN, a call to an Internet service provider occupies at least one B channel having a bit rate of 64 kbit/s but data is transmitted from the Internet service provider to the subscriber terminal at an average bit rate in the order of 5 kbit/s.
Whatever type of transmission is used between the subscriber terminal and the Internet service provider, the resources of the telephone network are therefore under-utilized. Also, Internet traffic is expanding fast. There is therefore a risk of telephone networks becoming congested by Internet traffic.